1. Field of the Invention:
The invention relates to a process for preparing polyorganosiloxanes by reaction, preferably through condensation, low molecular polyorganosiloxanols with each other or with oligomeric siloxanes in the presence of a known catalyst system. These products are silicone oils and silicone rubber polymers or starting materials for further modifications.
2. Discussion of the Background:
During the hydrolysis of organochlorosilanes, oligomeric siloxanes having both cyclic and linear or cross-linked structures are formed. For example, during the hydrolysis of diorganodichlorosilane mixtures comprising diorganocyclosiloxanes and linear .alpha., .omega.-dihydroxypolydiorganosiloxanes are produced. The mixtures or their components are reacted into polymeric linear and/or cross-linked organosiloxanes as a function of the application. The kind of reaction can be influenced by the raw materials that are added, the catalyst system that is used and the process parameters pressure and temperature.
These reactions are usually conducted discontinuously, in agitating reactors, whose mixing action has been improved by the use of spiral stirrers (DE 17 70 048) and dissolver stirrers. However, a drawback lies in the long reaction times needed, typically ranging from 1 to 5 hours.
Development in this field is in the direction of continuous processes. Cell reactors comprising several stacked cells (DE 27 05 563) and tubular reactors (DE 34 39 543) in which catalysts are used, are known.
The development of suitable equipment has also had, among other things, the goal of lowering the content of volatile components such as cyclic polysiloxanes, and optionally of removing condensation water that is formed at the same time. By introducing screw mixers, such as the Ko-Mixer, single screw mixers or twin screw extruders (DE 17 19 321, DE 22 29 514, DE 39 14 912) for the aforementioned processes, residence times ranging from 2 to 13 minutes can be achieved owing to the more effective degassing possibilities and better exchange of heat and material.
The known processes for continuous production of polyorganosiloxanes exhibit the following drawbacks:
inadequate evaporation conditions due to a poor surface to volume ratio of the reaction mixture PA1 frequent foaming of the reaction mixture due to low gas volume in proportion to the quantity of reaction mixture in the reaction chamber PA1 wide molecular weight distribution, in the extreme case formation of gel particles, due to a wide residence time spectrum, which is caused by deposits in those parts of the reaction chamber around which there is no flow PA1 large quantities of nonuniform products that cannot be directly utilized and have intermediate viscosities due to the slow displacement of highly viscous products during the conversion from high to low specific viscosities PA1 different degrees of filling of the reaction chamber cause fluctuations in residence time and result in deviations from the targeted viscosity, thus leading to different viscosities in most of the conventional continuous processes PA1 remixing due to inadequate blocking of the conveyor members during the preparation of products with low viscosities, resulting in nonuniform polymers. PA1 very low content of volatile components in the product PA1 high uniformity (narrow molecular weight distribution) PA1 better space-time yields through shorter residence times as a consequence of better devolatization conditions for reaction products (e.g., condensation water) PA1 very good control of the process (filling level, foaming, stability, starting and leaving) PA1 remixing and thus related different molecular weight distribution are ruled out PA1 no quality restrictions due to gel particles caused by deposited products on the contact parts around which there is no flow PA1 possibility of virtually inertia-less control of the process with the goal of varying the viscosity of the final product by influencing the different process parameters (e.g., pressure) by means of a process viscometer. PA1 milder reaction conditions, which reduce the thermal stress on the products and prevent recycling and the PA1 reduction of the accumulation of intermediate fractions during product conversions by small quantities of the reaction mixture in the reaction chamber and good emptying possibilities.